A CPP-GMR head where GMR refers to a giant magnetoresistive effect is considered as one promising sensor to replace the conventional CIP (current in plane) GMR head for over 200 Gb/in2 recording density. GMR spin valve stacks typically have a configuration in which two ferromagnetic layers are separated by a non-magnetic conductive layer (spacer). One type of CPP-GMR sensor is called a metallic CPP-GMR that can be represented by the following configuration in which the spacer between the AP1 pinned layer and free layer is a copper layer and the following layers are sequentially formed on a substrate: Seed/AFM/AP2/Ru/AP1/Cu/free layer/capping layer. One of the ferromagnetic layers is a pinned layer in which the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) or pinning layer. The pinned layer may have a synthetic anti-parallel (SyAP) structure wherein an outer AP2 layer is separated from an inner AP1 layer by a coupling layer such as Ru. The second ferromagnetic layer is a free layer in which the magnetization vector can rotate in response to external magnetic fields. The rotation of magnetization in the free layer relative to the fixed layer magnetization generates a resistance change that is detected as a voltage change when a sense current is passed through the structure. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the stack. A lower resistance is detected when the magnetization directions of the free and pinned layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state.
In a typical CPP-GMR sensor, a bottom synthetic spin valve film stack which is generally represented as [seed/AFM/pinned/spacer/free/cap] is employed for biasing reasons and a CoFe/NiFe composite free layer is conventionally used following the tradition of CIP-GMR technology.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head. To meet this requirement, the CPP configuration is a stronger candidate than the CIP configuration which has been used in recent hard disk drives (HDDs). The CPP configuration is more desirable for ultra-high density applications because a stronger output signal is achieved as the sensor size decreases, and the magnetoresistive (MR) ratio is higher for a CPP configuration. An important characteristic of a GMR head is the MR ratio which is dR/R where dR is the change in resistance of the spin valve sensor and R is the resistance of the spin valve sensor before the change. A higher MR ratio is desired for improved sensitivity in the device and this result is achieved when electrons in the sense current spend more time within the magnetically active layers of the sensor. Interfacial scattering which is the specular reflection of electrons at the interfaces between layers in the sensor stack can improve the MR ratio and increase sensitivity. Unfortunately, the MR ratio is often very low (<5%) in many CPP-GMR spin valve structures involving metal spacers. A MR ratio of ≧10% and an RA of <0.5 ohm-um2 are desirable for advanced applications.
Another type of sensor is a so-called confining current path (CCP) CPP GMR sensor where the current through the Cu spacer is limited by the means of segregating metal path and oxide formation. With a CCP-CPP scheme, the Cu metal path is limited through an insulator template so that the MR ratio can be enhanced quite significantly. An example of a CCP-CPP GMR sensor has the following configuration: Seed/AFM/AP2/Ru/AP1/Cu/CCP layer/Cu/free layer/capping layer where the CCP layer is sandwiched between two copper layers. Typically, a CCP layer is formed by first growing an Al or AlCu layer on a Cu layer at the top of a crystalline stack of layers which results in rough surface morphology and large grain size with large distributions in the Al or AlCu film. In the ensuing pre-ion treatment (PIT) and ion-assisted oxidation (IAO) steps where Al or AlCu is exposed to oxygen to form a current confining path through Al2O3 and Cu segregation, it is inevitable that a rugged Al or AlCu layer leads to a non-uniform AlOx layer which means poor uniformity and a loss of control in device performance.
CCP layer formation is based on a well known fact that Al atoms have a different (higher) mobility than Cu atoms. After the PIT treatment, Al and Cu start to segregate from each other and when exposed to oxygen during the IAO step, Al attracts oxygen to form amorphous AlOx. Because Cu is more chemically inert to oxygen than Al under the process conditions, it tends to remain as a Cu metal phase and eventually forms a metal path.
In order for the CCP-CPP GMR approach to be widely accepted in manufacturing, a smoother CCP forming layer and one that has a morphology which enables more uniform metal paths to be formed during the PIT/IAO processes is required so that significant improvement in device uniformity can be achieved. A CCP forming layer is defined here as the one or more layers deposited on a Cu spacer which are subsequently transformed (with Cu) into the actual CCP layer as a result of the PIT and IAO processes.
During a routine search of the prior art, the following references were found. In U.S. Pat. No. 7,177,121, an amorphous metal layer made of an oxidized NiCr alloy or oxidized CoCr alloy is formed on the sides of a magnetoresistive element and beneath a magnetic domain control film, the magnetic characteristics of the magnetic domain control film are improved.
U.S. Patent Application Publication No. 2005/0094317 discloses a composite layer in a MTJ stack that is comprised of a central current control region and an insulating layer on either side of the central region. The central current control region is made of an oxide, nitride, or oxynitride of at least one of B, Si, Ge, Ta, W, Nb, Al, Mo, P, V, As, Sb, Zr, Ti, Zn, Pb, Th, Be, Cd, Sc, Y, Cr, Sn, Ga, In, Rh, Pd, Mg, Li, Ba, Ca, Sr, Mn, Fe, Co, Ni, Rb, and rare earth metals and may contain one type of metal such as Cu, Au, Ag, Pt, Pd, Ir, and Os.
U.S. Patent Application Publication No. 2003/0053269 describes a current confining layer made of Al2O3 or TaO2 that is formed between a pinned layer and a free layer.